The use of implantable medical devices has become increasingly commonplace as an effective method of monitoring the state and condition of a living body. An implantable medical device can be implanted within a human or an animal to monitor physiological parameters about the patient's wellbeing. By being implanted directly within the body, implantable medical devices can provide continuous monitoring of the patient's condition without requiring continuous onsite care by a caregiver or a physician. Implantable medical devices can also provide therapy within the body to change or improve the patient's physical state based on the physiological parameters received from sensors or the like. Implantable devices have been used to help treat a variety of physical disorders, such as heart disease, deafness, and diabetes with a large degree of success.
It is often desirable for such an implanted medical device to wirelessly communicate with a remote external device. For example, the implantable medical device may communicate the acquired physiological parameters to the external device for processing or display for other user output. The implantable medical device may also communicate to the remote device information about how the implantable medical device is configured, or the implantable medical device instructions for performing subsequent commands within the implantable medical device. Implantable medical devices typically use a predetermined frequency band to communicate information to and from the external device or programmer. One example of such a frequency band is the medical implant communication service (MICS) band, which operates between 402-405 MHz. The range of communication between the implantable medical device and the external device can be limited by a number of factors, including the limitations on the physical size of antennas that can be used within implantable device and signal loss due to transmission through the body of the patient.
The wireless communications to and from the implantable medical device are sent via the same frequency band, for example, the MICS frequency band. The MICS band can be split up into ten channels for transmission in the 402-405 MHz range. Regulations regarding the MICS band require the ten channels to be scanned through for the channel with the lowest ambient signal level to be transmitted on, or on the first available channel with an ambient signal below a given threshold. The scanning is typically performed by an external device and the selected channel is then communicated to the implantable medical device.
As wireless transmissions are sent between the implantable medical device and the external device, they can consume a significant amount of power during their operation. Implantable medical devices typically use an internal battery to power the device. The battery life or operational time that the implantable medical device can be used is an important factor in the design of the devices as a shortened battery life may require additional surgery to replace or recharge the device at an unwanted time for the patient. For this reason, it is desirable to reduce the power consumption within the implantable medical device to increase its time duration of operation.
Because of the power requirements needed to sustain an implantable medical device, some implantable medical devices use a sleep state where the device is kept in a low-current usage state. The implantable medical device periodically looks or “sniffs” for a wake-up signal from an external device. Upon receiving the wake-up signal, the implantable medical device is powered on to normal operation, which utilizes significantly more current than during the sleep state. Alternatively, a duty cycle mode can be used by an implantable medical device to achieve lower power consumption, where the device is turned on during operation for a short time period and turned off following operation. Power savings can be achieved by duty cycling, in that the implantable device is not continuously on.